1. Field of the Invention
The present invention relates to a light-emitting device, and more particularly to increasing the effective light emission area.
2. Description of the Related Art
Flat display panels are recently known to use organic electroluminescence (organic EL) elements. Organic EL elements are self emitting displays in which an electrical current is supplied to an organic EL layer, provided between an anode and a cathode, causing the organic EL layer to emit light. Also since a backlight is unnecessary, unlike LCDs, organic EL elements are expected to be the next major type of flat display panel. In particular, in the active matrix type of organic EL display in which a switching element is formed at each pixel, a larger and higher resolution screen is possible since display data can be stored at each respective pixel.
In this sort of organic EL display, the organic EL layer, which is an emissive layer, is usually formed by a vacuum evaporation process using a shadow mask on a circuit substrate.
FIG. 6 shows a typical pixel arrangement of a color organic EL display 1. The organic EL display 1 comprises a plurality of pixels 1a arranged in a matrix configuration. Each pixel 1a has a switching element, such as a thin-film transistor, and the switching element of each pixel is driven by a gate line corresponding to the row and a data line corresponding to a column so as to cause the organic EL element to emit light. Although the arrangement of the R pixels, the G pixels, and the B pixels is arbitrary, the R pixels, the G pixels, and the B pixels may be positioned linearly (stripe arrangement in columns) as shown in the figure, for example.
FIG. 7 is a detailed top view of the pixel 1a of FIG. 6. FIGS. 8A and 8B respectively show the cross-sectional views of A—A and B—B of FIG. 7. In both figures, the area enclosed by a gate line 51 extending in the row direction and a data line 52 extending in the column direction represents one pixel area la. Within this area are formed an n-channel thin-film transistor 13, a storage capacitor 70, and a p-channel thin-film transistor 42. An organic EL element 65, which is connected via a drain electrode 43d to the thin-film transistor 42, is further provided. Furthermore, a source of the thin-film transistor 42 is connected to a power line 53 via a source electrode 43s. 
An active layer 9 of the thin-film transistor 13 forms a pattern of a double-gate structure that passes twice underneath gate electrodes 11 protruding from the gate line 51. A drain of the thin-film transistor 13 is connected to the data line 52 via a drain electrode 16 and a source is connected to a gate 41 of the thin-film transistor 42 via the storage capacitor 70 and a bridge structure. The storage capacitor 70 is formed from an electrode 55 integrated with the active layer 9 and an SC line 54 connected to power Vsc.
As described in the foregoing, the drain of the thin-film transistor 42 is connected to an organic EL element 60. The organic EL element 60 is comprised from an anode (transparent electrode) 61 that is formed at each pixel on a planarization insulating film 17 on the thin-film transistors 13, 42, a cathode (metal electrode) 66 that is formed in common with the pixels on the topmost layer, and an organic layer 65 that is laminated between the anode 61 and the cathode 66. The anode 61 is comprised from a material such as ITO and is connected via the drain electrode 43d to the thin-film transistor 42. Furthermore, the organic layer 65 is comprised by sequentially laminating from the anode 61, a hole transport layer 62, an organic emissive layer 63, and an electron transport layer 64. Although the organic emissive layer 63 is composed of a different material for the R pixels, the G pixels, and the B pixels, a material containing BeBq2 that includes a Quinacridon derivative is used.
The above-mentioned components of each pixel are all laminated on a substrate 3. Namely, an insulating layer 4 is formed on the substrate 3, and thereon a semiconductor layer 9 is formed as a pattern. Gates 11, 41 are then formed via a gate oxidizing film 12 on the semiconductor 9. An interlayer insulating film 15 is formed on gates 11, 41, and the active layer 9, such as of poly-silicon, is connected to the anode 61 via a contact hole that is formed in the interlayer insulating film 15.
Furthermore, to form the organic EL element 60 on the transparent anode 61 that is formed at each pixel, the organic emissive layer 63 may be evaporated for each of the R pixels, G pixels, and B pixels using a shadow mask 2 having an opening 2a corresponding to each pixel as shown in FIG. 9. Since a current is mainly supplied to the region sandwiched by the anode 61 and the cathode 66, the organic emissive layer 63 is positioned then formed on the anode 61.
In this configuration, when a selection signal is output to the gate line 51, the thin-film transistor 13 turns on and the storage capacitor 70 is charged in accordance with the voltage value of the data signal that is impressed on the data line 52 at the time. The gate of the thin-film transistor 42 receives a voltage corresponding to the charge stored in the storage capacitor 70. This controls the current that is supplied to the organic EL element from the power line 53 so that the organic EL element emits light at an intensity that corresponds to the supplied current.
However, when the organic emissive layer 63 is formed by vacuum evaporation using the mask 2 provided with openings 2a corresponding to the pixels as shown in FIG. 9, the thickness of the organic emissive layer 63 becomes uneven due to the shadowing effect at the edges of the opening 2a, resulting in a problem where uniform emission characteristics can not be obtained.
FIG. 10 shows a partial enlarged view of the mask 2. The opening 2a of the mask 2 is formed to match the shape of the anode 61 so as to be positioned on the anode 61 of each pixel as described in the foregoing, and allows the evaporated organic luminescence material to pass through from a evaporation source that is secured to a predetermined position. The film thickness of the organic emissive layer 63 along the z direction in the figure is approximately uniform at a center 100 as shown in FIG. 11 and the film thickness at edges 102 of the opening 2a is less than the center due to the shadowing effect. This sort of unevenness in film thickness may result in a problem of uneven light emission or a decrease in the effective light emission area.